The invention is directed to a new process for the production of .beta.-mercaptopropionic acid derivatives of the general formula (I): EQU H--S--CH.sub.2 --CH.sub.2 --X (I),
where X is --CN, --COOH or preferably --COOR, where R is alkyl.
It is known to produce .beta.-mercaptopropionic acid derivatives by reacting .beta.-chloropropionic acid derivatives with alkali hydrogen sulfide or with thiourea. These syntheses, however, are not satisfactory industrially because they take a long time or only give poor yields (E. Reid, Organic Chemistry of Bivalent Sulfur, Volume I, page 451).
Furthermore, it is known to react acrylonitrile with hydrogen sulfide to form .beta.-mercaptopropionitrile. Hereby however, there must be used a considerable excess of hydrogen sulfide if yields of about 38% are to be produced. Besides it is necessary to carry out this reaction under pressure (Wordie U.S. Pat. No. 2,748,155).
Even in carrying out this process in the presence of a catalyst system consisting of a base and sulfur there cannot be eliminated the basic disadvantages of this process (German No. OS 2,034,172).
Finally it is known that a direct addition of hydrogen sulfide to an acrylic ester for the purpose of forming the mercaptopropionic acid ester is not possible since instead there is always obtained the thiodipropionate (Riddel, Monomeric Acrylic Esters, 1954, page 148). In order to avoid such a double addition it is known to react acrylic acid esters with alkali hydrogen sulfide using carbon disulfide as the reaction medium. In this reaction, however, there must also be used an organic solvent, for example, an alcohol. Thereby reference is made to the fact that to the greatest extent possible no water should be present in the reaction mixture, since otherwise there are preferably formed thiodipropionic acid esters. However, on an industrial scale it is very difficult to realize these water free conditions. Besides the working up is made difficult through the additional solvent, since as a rule it hinders the isolation of the mercapto derivatives from the acidified, aqueous reaction mixture and besides makes necessary a distillative separation of solvent and carbon disulfide (German Pat. No. 2,244,234).